Just like a car; Make, Model & Serial Number is the place to start. "A shotgun" is like saying "a car". It could be a Vega or it could be a Lamborgini. Who would know? Serial numbers on long-arms have only been required since 1969 in the US. Before that, some manufacturers used serial numbers on all their firearms and the records have been preserved. Others didn't serialize any, and some serialized certain models. Remington uses a date code separate from the serial number and Marlin date codes are part of the serial. Nearly all Winchester, Colt, Browning and Ithaca firearms can be dated by the serial number. But many of the lesser names either have no serial numbers or there are no records available so you may have to be satisfied to know the time period during which a particular model was made. There are a few people who have all the old catalogs and determined what year certain features were introduced by the manufacturer they are studing.